In a monolithic SAW convolver having a structure (piezo-electric film/semiconductor) or a structure (piezo-electric film/insulator/semiconductor) or an air gap type SAW convolver having a structure (piezo-electric film/air/insulator/semiconductor) the concolution efficiency (hereinbelow abbreviated to F.sub..tau.) depends, in general, on the bias voltage applied to the gate electrode. FIG. 2 shows an example of the relation between the bias voltage and F.sub..tau. for an SAW convolver having a structure (ZnO/SiO.sub.2 /Si). FIG. 2 shows also the relation between the bias voltage and the capacitance between the gate electrode and the ground (hereinbelow called simply C-V characteristics). These C-V characteristics play an important role in the operational principle of this invention and will be explained later.
FIG. 2 indicates that F.sub..tau. has a maximum value at a certain bias voltage V.sub.op. V.sub.op is the optimum biasas voltage for the convolver and it is desirable to operate the convolver always at this bias voltage.
However, when a bias voltage is applied to a convolver, in many cases interfacial energy levels at the interface of semiconductor/insulator, traps at the interface of insulator/piezo-electric film, traps in the piezo-electric film, etc. can capture electrons and holes and it can take a fairly long time to stabilize the operation of the convolver because of the time necessary for the capture or generation. Further, because of these traps or interfacial energy levels described above the relation between F.sub..tau. and the bias voltage can depend on the history of the bias voltage applied heretofore and thus, if a bias voltage different from V.sub.op has been applied thereto, the optimum bias voltage thereafter can shift with respect to V.sub.op. These phenomena are reported in an article entilted "A Detailed Theory of the Monolithic Zinc Oxide on Silicon Convolver" by B. T. Khuri-Yakub and G. S. Kino (IEEE Transaction on Sonics and Ultrasonics, Vol. SU-24, No. 1, January 1977 (USA), p. 34).
Because of these characteristics, when a convolver is started, actually a fairly long warming up time is necessary for F.sub..tau. to become sufficiently great, measured from the point of time where the bias voltage V.sub.op begins to be applied to the convolver.
Further, since V.sub.op varies generally depending on the temperature, it is difficult to operate the convolver always with the highest F.sub..tau. by the method, by which a constant bias is applied thereto.